The present invention relates to an active matrix type organic light emitting diode display, and particularly relates to a high-performance active matrix type organic light emitting diode display in which the coupling-out efficiency of emission from an organic electroluminescence layer is improved, and a method for manufacturing the same.
An organic light emitting diode (hereinafter referred to as “OLED”) display is a self-emitting device different from a liquid crystal display needing a backlight. Accordingly, the OLED display is characterized in that it is thin in profile, wide in viewing angle, high in response speed and hence excellent in movie display in comparison with the liquid crystal display. In recent years, OLED displays have been researched and developed actively, and announcements of product planning have been made extensively.
A fundamental structure of an OLED display is a sandwich structure in which an OLED emission layer is sandwiched between two electrodes. In this event, the electrode serving to couple out the light of the emission layer to the outside has to be transparent.
Active matrix type OLED displays are classified into bottom emission type devices in which light is coupled out on the transparent substrate side where a pixel circuit is formed, and top emission type devices in which emission is coupled out on the opposite side to a transparent substrate where a pixel circuit is formed, in order to avoid restriction on the open area ratio due to light shielding of the pixel circuit.
In the bottom emission type devices, the electrode holding the OLED emission layer on the substrate side where the pixel circuit is formed is a transparent electrode. In the top emission type devices, the electrode holding the OLED emission layer on the opposite side to the substrate where the pixel circuit is formed is a transparent electrode. Known materials of the transparent electrodes include Indium Tin Oxide materials and Indium Zinc Oxide materials which are also used as transparent electrodes in liquid crystal displays.
In the active matrix type OLED displays serving as bottom emission type devices, glass such as non-alkali glass is often used on the transparent substrate side where a pixel circuit is formed. In this event, according to understanding of classical optics, it is believed (for example, see the following Non-Patent Document 1) that about 80% of light generated in an OLED emission layer is confined in a substrate due to the total reflection angle between the glass and the air, so that only about 20% of the light can be coupled out to the atmosphere. Even when the luminous efficiency of the OLED emission layer is improved to enhance the luminance, there will be a problem in the coupling-out efficiency of emission to the outside of the substrate such that the display performance cannot be improved.
As means to solve such a problem, there has been proposed a technique in which the coupling-out efficiency can be improved by using a transparent electrode substrate having a low-refractive body made from silica aerogel lower in refractive index than a glass substrate as disclosed in the following Patent Document 1. In addition, there has been proposed a technique in which the coupling-out efficiency can be improved by forming a film of spin-on-glass materials having nano pores in the film and having a lower refractive index than a glass substrate (see the following Non-Patent Document 2). The following Patent Document 2 also discloses that a low-refractive layer is provided. Patent Document 2 says it is preferable that the density of the low-refractive layer is especially not higher than 0.4 g/cm3.
[Patent Document 1] JP-A-2001-202827
[Patent Document 2] JP-A-2003-195775
[Non-Patent Document 1] M.-H. Lu, Appl. Phys. Lett., v78, p1927 (2001)
[Non-Patent Document 2] T. Nakayama, et al., International Display Workshops 2002 (IDW′ 02) proceedings. p1163 (2002)